The present embodiments relate to determining a position of a device on a z-coordinate axis relative to a B0 field magnet of a magnetic resonance tomography unit.
Magnetic resonance tomography units are imaging devices that, to map an object under examination, align the nuclear spin of the object under examination with a strong external magnetic field and stimulate precession around this alignment by an alternating magnetic field. The precession or return of the spin stimulated from this in a low-energy state generates an alternating magnetic field that is received via antennae as a response.
With the aid of magnetic gradient fields, spatial encoding is imprinted on the signals, which subsequently enables assignment from the received signal to a volume element. The received signal is then evaluated, and three-dimensional imaging of the object under examination is provided.
To receive the signal, local antennae (e.g., local coils) that are arranged directly on the object under examination may be used to obtain a better signal to noise ratio. Therefore, a position of the local coils with regard to the remaining magnetic resonance tomography unit (e.g., the B0 field magnet) is not firmly defined and is to be recorded separately. This relates, for example, to the position in the direction of the z-axis. The z-axis usually passes horizontally and centrally through the opening of the B0 field magnet as a symmetry axis. Along the z-axis, the object under examination is moved via a mobile patient couch into an area of a homogenous B0 field inside a patient tunnel by the B0 field magnet.
While the position of the object under examination in the x-coordinate axis and y-coordinate axis running orthogonally to the z-coordinate axis is relatively defined by the couch and is also covered by the homogenous B0 field at the center of the B0 field magnet, this may only ever cover a small area of the object under examination in the direction of the z-axis. It is therefore particularly important to record the z-coordinate of the local coil.
However, there are also applications in which a particularly high level of precision is to be provided in terms of alignment. For example, in a combined MRT-PET scan, the radiation produced by the positrons is attenuated by the local coil. To be able to compensate for this effect in evaluation, the position of the local coil is to be recorded as precisely as possible in the beam path in the magnetic resonance tomography unit (e.g., in all three coordinate axes).
For example, the projection of a model with which the local coil is aligned in the scanning area by a light source is known. This is difficult in the scanning area due to restricted accessibility. The local coil is to be arranged as close to the body as possible and is therefore possibly covered with heated blankets.